Bacillus is a genus of Gram-positive bacteria and a member of the phylum Firmicutes. Endospores are produced under conditions of environmental stress, which allows the bacteria to persist in a dormant state for extended periods of time and accounts for their persistence and worldwide distribution. Some Bacillus species secrete large quantities of enzymes that are of value to industy. Others are pathogenic and secrete toxins and other cytopathic enzymes that function as virulence factors.
Bacillus anthracis infection, or anthrax, is a zoonotic disease that occurs naturally in wild and domestic herbivores (e.g., cattle, sheep, goats, camels, antelopes and horses). Anthrax continues to be an important disease of livestock in less developed countries where vaccination is not widespread. It is also considered to be one of the most probable threats against military or civilian targets because of its availability in the environment, ease of spore dissemination, and the rapid lethality that occurs if antibiotic treatment is not initiated promptly. Depending on the mode of transmission, anthrax may manifest as inhalational, gastrointestinal, or cutaneous, with inhalational anthrax having the highest case fatality rate of up to 97% if left untreated. The only human anthrax vaccine licensed in the United States is primarily directed at the protective antigen (PA) toxin component.
Bacillus cereus is a foodborne pathogen, causing gastrointestinal infections that tend to be self-limiting but occasionally may have more serious consequences such as fatal toxemia. Many other Bacillus species, particularly B. sustilis and B. licheniformin, have also been incriminated periodically as agents of food poisoning. Skin infections and keratitis have also been resported. However, B. cereus has become increasingly recognized as a source of serious and potentially fatal extraintestinal infections including progressive pneumonia, sepsis, and central nervous system infections, particularly in immunocompromised individuals. Nosocomial infections are of particular concern, especially given the ability of B. cereus to permeate and persist in the hospital environment, and outbreaks among hospitalized patients are well-documented.
Bacillus thuringiensis is an insecticidal pathogen that has been used extensively as a biopesticide. Although infection of humans is rare, cases have been documented. Because B. thuringiensis and B. cereus are closely related and appear to be able to exchange genetic material on plasmids, there are many unresolved questions concerning the safety of exposure to aerosolized B. thuringiensis during large-scale insect eradication operations.
Although Bacillus anthracis, B. cereus and B. thuringiensis are well defined and remain the best-known. Other species of Bacillus have been increasingly implicated in a wide range of infections in mammals and humans including abscesses, bacteremia/septicemia, ear infections, endocarditis, wound and burn infections, meningitis, ophthalmitis, osteomyelitis, peritonitis and respiratory and urinary tract infections. Most of these occur as secondary infections or mixed infections often in an immunodeficient or immunocompromised host, but a significant proportion are now being reported as primary infections in healthy individuals generating severe and often lethal outcomes. Most frequently implicated in these types of infections are B. cereus, B. licheniformis, and B. subtilis, although B. brevis, B. alvei, B. circulars, B. coagulans, B. macerans, B. pumilus, B. sphaericus and B. thuringiensis occasionally cause infections. As secondary invaders, Bacillus species can exacerbate preexisting infections by producing tissue damaging toxins or metabolites that have been shown to interfere with treatment.